Diborane

Since we are primarily concerned with kinetics, we do not intend to discuss all the proposed mechanisms or the numerous boron hydrides formed during the pyrolyses. The literature available on this field up to 1964 has been reviewed by Adams81 and by Lipscomb85. Rapid progress is being made in studies of the kinetics of boron hydride decomposition so that this discussion must be regarded as preliminary. 

Assuming equilibrium between B2H6 and BH3 and a steady-state concentration for the intermediate hydrides , this gives a rate expression 

In the initial stage of the reaction, where the concentration of hydrogen is negligible, this reduces to the f-order rate law. Later in the decomposition, however, it is equivalent to the empirical rate law only if the expression 

At about the same time, the pyrolysis of diborane was studied by Bragg et al.88 in the temperature range 90-130 °C. These workers again used a static system (reaction vessel volume 212 cm3) and followed the conversion both by measurement of pressure increase and by determination of the amount of hydrogen formed. The system was also examined by mass spectrometric analysis. The empirical rate law was found to be 

The mass spectra showed BsHn to be an intermediate and B5H9 a relatively stable product, and thus Bragg et al. proposed the mechanism 

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The deposition of amoriDhous hydrogenated silicon (a-Si H) from a silane plasma doped witli diborane (B2 Hg) or phosphine (PH ) to produce p-type or n-type silicon is important in tlie semiconductor industry. The plasma process produces films witli a much lower defect density in comparison witli deposition by sputtering or evaporation. 

The structure of diborane B2H6 is considered later (p. 145). Here we may note that "BHj and "AlHj will be acceptor molecules since there are only six valency electrons around the B or A1 atom and a vacant orbital exists. Both in fact can accept the electron pair from a hydride ion thus ... 

Pure boron in the form of a thin film can also be obtained by heating diborane to 1000 K ... 

Boron forms a whole series of hydrides. The simplest of these is diborane, BjH. It may be prepared by the reduction of boron trichloride in ether by lithium aluminium hydride. This is a general method for the preparation of non-metallic hydrides. 

Diborane has a qeometric structure similar to that of dimeric aluminium chloride, namely... 

This is known as a hydrogen-bridge structure. There are not enough electrons to make all the dotted-line bonds electron-pairs and hence it is an example of an electron-deficient compound. The structure of diborane may be alternatively shown as drawn in... 

Borane does not exist as such, but a donor molecule can break up diborane and form an adduct, thus ... 

There is one other important way in which borane can be stabilised. Diborane reacts with a suspension of lithium hydride in dry ether thus... 

Aluminium tetrahydridoborate is a volatile liquid. It is the most volatile aluminium compound known. It is covalent and does not contain ions but has a hydrogen-bridge structure like that of diborane, i.e. each boron atom is attached to the aluminium by two hydrogen bridges ... 

Other boron hydrides are known, most of them having the general formula B H + 4- for example pentaborane, B5H9, decaborane, BjqHi4. Each can be made by heating diborane in suitable conditions for example at 420 K, decaborane is obtained. Boron hydrides have been tried as rocket fuels. 

Aluminium hydride loses hydrogen on heating. It reacts slowly with diborane to give aluminium tetrahydridoborate ... 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Figure 2-60. Soine examples of electron-deficient bonds a) diborane featuring B-H-B bonds b) diborane in a tentative RAMSES representation c) the orbital in a B-B-B bond (which occurs in boron cage compounds),...

TrialkyIboranes (p. 9), which can be synthesized from olefins and diborane, undergo alkyl coupling on oxidation with alkaline silver nitrate via short-lived silver organyls. Two out of three alkyl substituents are coupled in this reaction. Terminal olefins may be coupled by this reaction sequence in 40 - 80% yield. With non-terminal olefins yields drop to 30 - 50% (H.C. Brown, 1972C, 1975). 

Diborane or aUcylboranes are used for reduaion of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

If it is necessary to reduce one group in a given molecule without affecting any other unprotected reducible group, the following reactivity orders for ease of reduction toward catalytic hydrogenation, LiAlH, and diborane may serve as a guideline. 

Synthesis by high-dilution techniques requires slow admixture of reagents ( 8-24 hrs) or very large volumes of solvents 100 1/mmol). Fast reactions can also be carried out in suitable flow cells (J.L. Dye, 1973). High dilution conditions have been used in the dilactam formation from l,8-diamino-3,6-dioxaoctane and 3,6-dioxaoctanedioyl dichloride in benzene. The amide groups were reduced with lithium aluminum hydride, and a second cyclization with the same dichloride was then carried out. The new bicyclic compound was reduced with diborane. This ligand envelops metal ions completely and is therefore called a cryptand (B. Dietrich, 1969). 

Diborane is the more electrophilic of the two reagents and while LiAH-14 often... 

A solution of 1.05 M diborane in THF (25 ml, 26 mraol) was added slowly to a stirred suspension of 3-acetyl-5-hydroxy-2-methylindole (1.0 g, 5.3 mmol) in THF (10 ml). After hydrogen evolution ceased, the mixture was heated at reflux for I h, cooled and poured into acetone (75 ml). The mixture was heated briefly to boiling and then evaporated in vacuo. The residue was heated with methanol (50ml) for 20min. The solution was concentrated and 3NHC1 (40ml) was added. The mixture was extracted with ether and the extracts dried (MgSO ) and evaporated to yield a yellow oil. Vacuum sublimation or recrystallization yielded pure product (0.76 g, 82%). 

There is a pronounced tendency for boron to become bonded to the less substituted car bon of the double bond Thus the hydrogen atoms of diborane add to C 2 of 1 decene and boron to C 1 This is believed to be mainly a steric effect but the regioselectivity of addition does correspond to Markovmkov s rule m the sense that hydrogen is the neg atively polarized atom m a B—H bond and boron the positively polarized one... 

Borane (BH3) does not exist as such under normal condi tions of temperature and at mospheric pressure Two molecules of BH3 combine to give diborane (B2H6) which IS the more stable form... 

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

Examples BjHg, diborane Bj(,Hj4, decaborane(14) BjqHjj, decaborane(16) P2H4, diphos-phane SnjHg, distannane HjSej, diselane HjTej, ditellane H2S5, pentasulfane and PbH4, plum-... 

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